Method of making a photosensitive semi-conductor device



Dec. 6, 1960 D. c. DICKSON, JR 2,963,390

METHOD OF MAKING A PHOTO-SENSITIVE SEMI-CONDUCTORDEVICE Filed Sept. 26,1955 ALD c. DICK ,JR.

ATTORNEYS IN VE 3 WWMQWJQM Y United States Patent METHUD OF MAKING APHOTOSENSITIV E SEMI-CGNDU CTOR "DEVICE Donald C. Dickson, Jr., ProspectHeights, Ill., assignor to Hoffman Electronics Corporation, Los Angeles,Califl, acorporation of California Filed Sept. 26, 1955, Ser. No.536,489

23Claims. (Cl. 148-1.-5)

The present invention relates, in general, to electronics andzhas moreparticular reference tophoto'sensitive electron flow =devices employingsemi-conductor material, the invention pertaining specificallyto animproved-photosensitivedevice embodying a P N junction.

A semiaconductor material .may comprise an intrinsic or :pure substance,such as crystalline germanium, silicon or other suitable material, inwhich the valence ring electrons .ofthe constituent-atoms are tightlybound-and hencesunavailable for electrical conduction, the intrinsicmaterial being doped or contaminated with an activating substancecomprising atoms distributed in and forming an integral part of thelattice structure of the intrinsic material. The proportion ofactivating impurity to intrinsic. material may boot the order of oneimpurity atom for each hundred million atoms of intrinsic material.

Suitable activating substances for activating intrinsic semi-conductormaterial comprise atoms containing either a=-greater or lesser number ofvalence electrons than the constituent atoms of the intrinsic material.Where the atoms of the activating substance comprise more valenceelectrons than are associated with the atoms of the intrinsic material,the excess electrons may circulate freely in and through the crystalstructure of the semi-conductor material and are hence available forelectrical conduction purposes. Activating substances which provideexcess electrons are. commonly referred to as electron donors sincetheysupply electrons which are free to move within the lattice structure ofthe semi-conductor material. Excess electron semi-conductors arecommonly referred *to as comprising N-type material since electricalconduction is carried on by the flow of negatively charged electronsthrough the material. 7

Where the atoms of the activating substance embody fewersvalenceelectrons than are associated with the'atoms of :.the intrinsicsubstance, each atom ofactivating material must borrow electrons'from anadjacent atom of the intrinsic material, thereby creating-what maybereferred to as ahole; that is to say, an incomplete group ofatom bondingelectrons simulating the properties of a positivelycharged electron inthe lattice structure of the semi-conductor material. Hole creatingactivators are commonly called acceptormaterials since they take upelectrons-from the surrounding atoms of the intrinsic material to formpositive holestherein. Intrinsic material activated by an acceptorsubstance is commonly referred towas aP-type semi-conductor sinceconduction therein is effected by the movement of positive holes in thematerial.

Electricalrenergy may flow .in. either direction through both types ofsemi-conductor material. When, however, a; bodyof P-type material isjoined with a body of N- typelmaterial to form therebetween what iscommonly referred tons a -P-N junction,-the positive holes in the P-typematerial and the free electrons in the N-type material-are mutually.repelled away from the junction. In this. connection, the. P-N-junction :is theequivalent .of a

2,963,390 Patented Dec. 6, 1960 unidirectional potential source havingnegative and positive sides respectively connected with the P-type andN- type materials on opposite sides of the junction, therebyconstituting the junction as a potential hill, past which electricalenergy may flow more easily in one direction than the other. If, forexample, the positive and negative sides of a source of unidirectionalelectrical potential be connected respectively with the N-type andP-type ends of a P-N junction element, in fashion commonly referred toas reverse bias, such potential source will merely increase thepotential hill, by drawing the valence electrons and holes mutuallyawayfrom the junction, and, consequently no current flowmay take placeacross the P-N junction.

On the .otherhand, connection of the positive and negative sides of asource of unidirectionalelectrical potential, as a so called forwardbias, respectively to the P-type and N-type sides of a P-N junctionelement, will tend to offset the potential hill "by'driving the holesoftheP-type material-and the electrons of the N-type material toward thejunction, thereby allowing current flow across the junction if and whenthe applied potential exceeds the potential drop across the junctionelement, including the equivalent potential value of the junction hill.Accordingly, a semi-conductor element embodying a PN junction may beemployed as an electrical power rectifier.

Heretofore semi-conductor elements embodying P-N junctions have beenproduced by initially growing a crystal ingot of intrinsic material todesired size in the presence of an activator of one' kind, an electrondonor material, for example, to thereby constitute the resultingcrystalline substance as N-type semi-conductor material. Thereaftergrowth of the crystal ingot may be continued in the presence of anactivating substance of the other kind, such as an electron acceptormaterial, to thereby constitute the subsequently grown portions of theingot as P-type material. Insuch an ingot, the P-N junction extendsbetween the N-type and P-type portions of the ingot body.

Semi-conductor elements embodying P-N junctions may show photosensitivecharacteristics where a junction is disposed close toa surface adaptedfor exposure to light.- When light impinges upon such ajunction-adjacent surface of a semi-conductor element, electron-holepairs will be created in the semi-conductor element at the surface uponwhich light is impinged, such electron-hole pairs being initially underno electrical stress. The junction resistance is such that substantiallyall of the applied potential difference appears across the junction,there being little, if any, voltage drop across the junction formingbodies of semi-condu'ctormaterial on opposite sides of'the junction. Asa consequence, the electron =hole pairs created as thezresult of lightapplied to the element will diffuse through the semi-conductor materialand some of them will come into the surface adjacent junction area. Byso doing, the junction pairs lower the resistance of the junction area,at least temporarily, thereby causing current flow therein which will bedirectly proportional to the intensity of impinging light, since thenumber of electron-hole pairs created will be a function of the amountof light applied to the semiconductor element.

An important object of the present invention is to provide aphotosensitive device of the character mentioned having anunusuallylarge light receiving surface, thereby affording highlysensitive light responsive characteristics.

Another important object of the present invention is to provide animproved photosensitive semi-conductor of .thecharacter mentioned byapplying, to a body of material of selected type, an activatingsubstance .in vaporized condition, thereby to conditionportionsof saidbody at and inwardly of a light receiving surface thereof as material ofanother type, whereby to form a P-N junction between the so conditionedsurface portions of the body and the portions thereof lying inwardly ofsuch surface portions, said junction being closely adjacent the saidlight receiving surface of said body; a further object being to formsaid body as a relatively thin layer or plate and to condition saidlayer in fashion forming a P-N junction thereon at a depth of the orderof of an inch behind a light receiving surface of the layer; a stillfurther object being to provide a layer of semi-conductor materialhaving thickness of the order of of an inch and conditioned to provide aP-N junction medially between the opposite faces of the layer withP-type material on one side of the junction, and N-type material on theother; yet another object being to mount the layer upon a supportingbase plate of suitable electrical conducting material such as copper,nickel or tantalum, preferably having a thickness of the order of A5" orless, the layer of semi-conductor material being electrically connectedin ohmic fashion on said base plate throughout a surface of said layer;yet another object being to ohmically connect the layer ofsemi-conductor material upon the support base prior to the exposure ofthe base remote, light receiving surface of the layer to the vaporizedactivating substance for the formation of the P-N junction therein.

Another important object is to provide a photosensitive semi-conductordevice comprising a body of silicon of selected type having portions atand inwardly of a light receiving surface thereof, treated with anactivating substance to condition said portions as material of adifferent type, thereby forming a P-N junction between the soconditioned surface portions of the body and the portions thereofdisposed inwardly of and closely adjacent light receiving surface; afurther object being to form the layer of silicon of selected type upona plate or panel of electron conducting material by vaporization andcondensation of the silicon to desired depth upon the panel; yet anotherobject being to first coat the support panel of conducting materialprior to the application of the layer of semi-conducting materialthereon with a thin bonding layer of a substance capable of formingohmic connection between the support panel and the layer ofsemiconducting material.

A further object of the invention is to provide a photosensitive devicecomprising a relatively thin layer of N-type silicon having portions ofsaid layer at and inwardly of a light receiving surface thereof, dopedwith boron, aluminum, gallium, indium, thallium or other suitableelectron acceptor material, in order to constitute such doped portionsof the layer as P-type material; a further object being to secure thelayer of N-type silicon upon a support plate or panel of electricalconducting material such as copper, nickel, tantalum, and the like, bymeans of a thin bonding layer of gold doped with antimony or otherbonding substance capable of forming substantially ohmic connectionbetween the N-type silicon layer and the support panel; a still furtherobject being to apply the bonding layer to a depth of the order of of aninch upon the support panel behind'the layer of semi-conductingmaterial.

A further object of the invention is to provide a photosensitive devicecomprising a relatively thin layer of P-type silicon having portions ofsaid layer at and inwardly of a light receiving surface thereof, dopedwith phosphorus, arsenic, antimony, bismuth or other suitable electrondonor material, in order to constitute such doped portions as N-typematerial; a further object being to secure the layer of P-type siliconupon a support plate or panel of electrical conducting material such ascopper, nickel, tantalum, and the like, by means of a thin bonding layerof aluminum or other suitable bonding substance capable of formingsubstantially ohmic connection between the P-type layer and thesupportpanel; a still further object being to apply the bonding layer to adepth of the order of of an inch upon the support panel behind the layerof semi-conducting material.

The foregoing and numerous other important objects, advantages andinherent functions of the invention will become apparent as the same ismore fully understood from the following description, which, taken inconnection with the accompanying drawings, discloses preferredembodiments of the invention.

Referring to the drawings:

Fig. 1 is a perspective view of a photosensitive semiconductor deviceembodying the present invention;

Fig. 2 is an enlarged and partially sectionalized view of the deviceshown in Fig. 1;

Fig. 3 is a greatly enlarged sectional view taken substantially alongthe line 3-3 in Fig. 1; and

Figs. 4 and 5 are face views of modified photosensitive semi-conductordevice embodying the present invention.

To illustrate the invention, the drawings show a photosensitivesemi-conductor device 11 comprising a relatively thin layer 12 ofsemi-conductor material providing a P-N junction 13 between the oppositefaces of the layer, one surface of said layer being preferably ohmicallyconnected upon a support plate 14 of electrical conducting material, andthe opposite surface of the layer 12 being exposed for light impingementthereon. While the broader aspects of the invention are not necessarilylimited to the specific semi-conductor material of the layer 12, theinvention, in its specific aspects, contemplates the use of silicon asthe constituent material of the layer 12. Photosensitive characteristicsare imparted to the layer 12 by disposing the P-N junction 13exceedingly close to the light receiving surface of the layer; and it isan important object of the present invention, not only to provide forthe disposition of a P-N junction close to the light receiving surfaceof the layer, but also to provide a junction of substantial area and toform the same at a distance behind the light receiving surface of thelayer that is substantially uniform throughout its entire area.

In order to accomplish the objectives of the present invention, thesupport plate 14 may comprise a panel or disc having thickness of theorder of A3 inch, or less. The disc may be of any suitable or convenientsize and configuration, a circle having diameter of the order of oneinch or more being suitable for the intended purpose. The layer 12, asinitially applied to the base plate 14, may comprise either P-type orN-type semi-conductor material, such as silicon, the N-type materialbeing particularly well suited for use in producing large areaphotosensitive devices, in accordance with the present invention. Ineither case, the base plate 14 may conveniently comprise tantalum,copper, nickel, or other self-supporting electron conducting material.

In order to secure the layer 12 upon the base plate 14, a thin film ofbonding material may first be applied upon the layer carrying surface ofthe base plate, said film of bonding material serving to form a strongmechanical bond, as well as an ohmic, or low resistance, electricalconnection between the layer 12 and the support plate throughout theentire area of the supported layer. The bonding layer 15 may havethickness of the order of 0.00001 inch and may be applied as byvaporization and condensation of the bonding material upon the surfaceof the support plate. Where the layer 12 initially consists of N-typesemi-conductor material, the bonding layer 15 may comprise gold dopedwith antimony or other suitable electron donor material; but where saidlayer 12 initially consists of P-type material, the bonding layer 15 maycomprise aluminum or other electron acceptor type materia Afterapplication of the bonding film, the layer 12 of semi-conductor materialmay be applied as by depositing silicon of the selected type upon thefilm 15 to a depth of the order of 0.0002 inch. The material of thelayer 12 may conveniently be applied by vaporization and condensation ofthe selected material upon the bonding film 15, asby exposing the coatedsurface of the supportplate 14in an atmosphei'e comprising the selectedsemi-conductor material in vaporized condition for a lengthof timesuflicient to build up the layer 12. to desired depth upon the coatedsurface 'ofthe support plate.

After the layer-12 "has thus been applied upon, and-in ohmic contactwith, the supportplate 14, it may be treated to alter the type of itsmaterial at and to a desired depth inwardly ofiiitslightreceiving.surface, as in the o r portions 16 of the layer, the inner portions 17of the layer remaining unchanged, whereby to form the P-N junction 13between the layer portions 16 and 17 which are thus constituted aslaminae of material of unlike type, on opposite sides of the P-Njunction 13. In order to so change the type of material in the outerportions, or lamina 16 of the layer 12, it may be treated with anactivating substance other than, and different from, that which isdistributed in the material, in order to determine its initial type.Accordingly, where the layer 12 comprises N-type semi-conductormaterial, the layer portions 16 at and inwardly of its light receivingsurface may be conditioned with boron, aluminum, indium, gallium,thallium, or other suitable electron acceptor substance, in order toconstitute the layer portions 16 as P-type semi-conductor material.Conversely, where the layer 12 initially comprises P-type semi-conductormaterial, the portions 16 may be treated with arsenic, antimony,bismuth, or other electron donor substance, in order to constitute thelayer portions 16 as N-type semi-conductor material. The layer 12,preferably, consists initially of N-type silicon embodying arsenic,antimony, bismuth, or other electron donor substance, and the surfaceportions 16 being treated with boron or other electron acceptorsubstance, to constitute the same as P-type semi-conductor material.

The activating or conditioning substance employed to determine thecharacter of the material in the layer p ortions 16 may be applied tothe light receiving surface of the layer, and caused to penetrate to adesired depth therein by exposing the layer, in situ, on the supportplate 14, in an atmosphere consisting of the selected activatingsubstance in vaporized condition, the plate supported layer beingretained in such atmosphere and hence soaked in the vaporized activatingsubstance during a period of sufficient duration to obtain penetrationof the activating substance to a desired depth at and beneath the lightreceiving surface of the layer. It will be apparent, of course, that theP-N junction 13 Will be established between the layer portions 16 and 17of contrasting type.

In order to provide for electrical connection of the layer portions 16and 17 in external electrical circuits, electrode means 18 may beelectrically connected with the layer portion 16, while a connectionterminal 19, such as a threaded post, may be soldered, or otherwiseelectrically connected, with the base plate 14. The terminal 19 mayconveniently be secured to the one side of the support plate 14 remotefrom the layer 12, the terminal being preferably attached at the medialportions of the support plate.

The electrode means 18 may conveniently comprise nickel, electroplatedto a depth of the order of 0.0005 inch, preferably in the form of anannular ring 20 deposited on, and in electrical contact with, theperipheral edges of the layer portion 16, such ring affording means forattachment as by soldering or otherwise with a connection conductor 21for connecting the layer portion 16 in external circuitry. The foregoingarrangement for making electrical contact with the layer portion 16 isan important feature of the invention, since it is not easy to connect aconductor directly with a thin layer of material such as the portion 16,the thickness of which is of the order of 0.0001 inch.

The device described may be employed as a photosensitive device bymounting the same in position to receive the impingement of light uponthe outwardly facing surface of the layer portion 16. Where lightimpinges up,o n,,such surfa e, lectr n ql rair wi1 1.,.b.

hole pairslwilltdiffuse-through the semi-conductor materialof the layerportion 16 andinto the junction area, thereby lowering the resistance ofthe junction and allowing current tlow there'through, in amountsubstantially proportional to the intensity of impinging light.

While the ring like electrode 20 shown in Fig. 1 is adequate formostpurposes, itmayube desirable to .provide the electrode in "the form of alaminous network 22 of conductive material applied upon the lightreceiving surface of the layer portion 16, which network may comprise aring-like portion 20', similar to the ring-like electrode 20 of thedevice as shown in Figs. 1, 2 and 3, and a plurality of preferablysubstantially equally spaced apart, line-like fingers 23 of electricalconducting material, electrically interconnected and connected with thering-like portion 20, said line-like fingers being interspersed inspaced apart relation substantially throughout the surface of the layer16, within the ring-like electrode portion 20'. As shown moreparticularly in Fig. 4 of the drawings, the fingers 23 may be of curvedconfiguration and may be electrically interconnected and connected withthe ring-like electrode portion 20 by means of radially extendingconnecter portions 24. As shown in Fig. 5, the fingers 23 of electricalconducting material may be relatively straight and connected directlywith the ring-like portion 20'. Any other preferred pattern orarrangement of spaced fingers may, of course, be employed. The fingers23 and the connecting portions 24 may comprise the same material that isused in forming the ring-like electrode 20 and the electrode portion20', which material may be applied by electroplating the same in adesired pattern upon the light receiving surface of the layer portion 16simultaneously with the application of the ring-like portion 20'.

The foregoing procedure for the fabrication of a photosensitivesemi-conductor device permits the fabrication of a device having anydesired, precisely determined area for receiving the impingement oflight. The foregoing procedure, furthermore, allows for exceedinglyprecise control, not only of the thickness of the layer 12, but also thespacement of the junction 13, with respect to the light receivingsurface of the layer 12. The resulting device is of exceedingly ruggedcharacter which cannot be destroyed by any usual shocks or jars to whichelectronic equipment is ordinarily subjected in service.

It is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction andarrangement of the several parts without departing from the spirit andscope of the invention, or sacrificing any of its attendant advantages,the forms herein disclosed being preferred embodiments for the purposeof illustrating the invention.

The invention is claimed as follows:

1. The method of making a light sensitive semi-conductor device, whichconsists in applying a thin layer of bonding material to a surface of asupport plate of electrical conducting material, then depositing, byvaporization and condensation, a layer of crystalline semiconductormaterial of selected conductivity type throughout and having a thicknessof approximately 0.0002 inch upon said layer of bonding material, saidbonding material being of the type to form an ohmic contact between saidlayer of semi-conductor material and said support plate, thereafterexposing said layer of semiconductor material to the action of anactivating substance in vaporized condition to penetrate the layer withthe substance at and to a desired depth approximating 0.0001 inchinwardly of the plate-remote surface of said layer, whereby toconstitute the portion of said layer of semi-conductor material at andinwardly of said plateremote surface as material of differentconductivity type,

a r 7 and to form a P-N junction between the substance penetrated andremaining portions of said layer. 1

2. The method in accordance with claim 1, in which said bonding materialis applied by vaporization and condensation to a thickness approximating0.00001 inch on 5 the surface of the support plate.

References Cited in the file of this patent UNITED STATES PATENTS2,567,970 Scalf et a1 Sept. 18, 1951 10 8 Pearson "Feb, 24, 1953 DunlapJuly 7, 1953 Haynes Oct. 12, 1954 Conrad May 3, 1955 Hewlett Feb. 5,1957 FOREIGN PATENTS Belgium Nov. 14, 1952

1. THE METHOD OF MAKING A LIGHT SENSITIVE SEMI-CONDUCTOR DEVICE, WHICHCONSISTS IN APPLYING A THIN LAYER OF BONDING MATERIAL TO A SURFACE OF ASUPPORT PLATE OF ELECTRICAL CONDUCTING MATERIAL, THEN DEPOSITING, BYVAPORIZATION AND CONDENSATION, A LAYER OF CRYSTALLINE SEMICONDUCTORMATERIAL OF SELECTED CONDUCTIVITY TYPE THROUGHOUT AND HAVING A THICKNESSOF APPROXIMATELY 0.0002 INCH UPON SAID LAYER OF BONDING MATERIAL, SAIDBONDING MATERIAL BEING OF THE TYPE TO FORM AN OHMIC CONTACT BE TWEENSAID LAYER OF SEMI-CONDUCTOR MATERIAL AND SAID SUPPORT PLATE, THEREAFTEREXPOSING SAID LAYER OF SEMICONDUCTOR MATERIAL TO THE ACTION OF ANACTIVATING SUBSTANCE IN VAPORIZED CONDITION TO PENETRATE THE LAYER WITHTHE SUBSTANCE AT AND TO A DESIRED DEPTH APPROXIMATING 0.0001 INCHINWARDLY OF THE PLATE-REMOTE SURFACE OF SAID LAYER, WHEREBY TO CONSITUTETHE PORTION OF SAID LAYER OF SEMI-CONDUCTOR MATERIAL AT AND INWARDLY OFSAID PLATEREMOTE SURFACE AS MATERIAL OF DIFFERENT CONDUCTIVITY TYPE, ANDTO FORM A P-N JUNCTION BETWEEN THE SUBSTANCE PENETRATED AND REMAININGPORTIONS OF SAID LAYER.